1. Field of the Invention
This invention relates to a heterojunction bipolar transistor (HBT) and method of manufacturing an HBT, and more particularly to an HBT formed by organometallic vapor phase epitaxy (OMVPE) using zinc as the base dopant.
2. Discussion of the Background
A considerable amount of testing (qualification) with respect to, for example, the reliability and performance of heterojunction bipolar transistor (HBT) structures is needed prior to the device entering the manufacturing stage.
Some of the methods used relating to the growth of HBT structures include molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE).
FIG. 1 illustrates a background HBT epitaxy device structure grown by MBE or OMVPE. In FIG. 1, layer 1 is an n-type (n+) doped GaAs collector contact with a doping concentration between 2.times.10.sup.18 -1.times.10.sup.19 cm.sup.-3 and a thickness between 4,000-7,000 .ANG. grown on a GaAs substrate S, layer 2 is an n-type (n-) doped GaAs collector transit layer with a doping concentration between 7.times.10.sup.15 -5.times.10.sup.16 cm.sup.-3 and a thickness between 4,000-15,000 .ANG., and layer 3 is a p-type (p+) doped GaAs base with a doping concentration between 1.times.10.sup.19 -1.times.10.sup.20 cm.sup.-3 and a thickness between 500-1,000 .ANG.. In addition, layer 4 is an n-type (n) doped Al.sub.x Ga.sub.(1-x) As emitter (0.2&lt;x&lt;0.4) with a doping concentration between 1.times.10.sup.17 -1.times.10.sup.18 cm.sup.-3 and a thickness between 300-2,000 .ANG., layer 5 is an n-type (n+) doped Al.sub.x Ga.sub.(1-x) As graded (0.2&lt;x&lt;0.4 linearly graded to 0.0) layer with a doping concentration between 1.times.10.sup.18 -1.times.10.sup.19 cm.sup.-3 and a thickness between 200-500 .ANG., and layer 6 is an n-type (n+) doped emitter contact with a doping concentration between 1.times.10.sup.18 -1.times.10.sup.19 cm.sup.-3 and a thickness between 500-3,000 .ANG..
Many modifications of this structure are possible, such as the emitter contact layer 6 including combinations of GaAs and In.sub.y Ga.sub.(1-y) As. For example, the emitter contact may include In.sub.y Ga.sub.(1-y) As (0.4&lt;y&lt;0.6) with a doping concentration between 5.times.10.sup.18 -1.times.10.sup.19 cm.sup.-3 and a thickness between 500-1,000 .ANG. or the emitter contact may include n+ GaAs with a doping concentration between 1.times.10.sup.18 -1.times.10.sup.19 cm.sup.-3 and a thickness between 500-2,000 .ANG.. In addition, an n+ In.sub.y Ga.sub.(1-y) As graded region (0.4&lt;y&lt;0.6 linearly graded to 0.0) with a doping concentration between 5.times.10.sup.18 -1.times.10.sup.19 cm.sup.-3 and a thickness between 500-1,000 .ANG. may be used. A further modification is that the AlGaAs emitter (layer 4) may be replaced with GaInP or an n-type (n) doped AlGaAs graded layer may be used between the emitter and base to reduce the turn-on voltage. Still further, the p+ GaAs base (layer 3) may include AlGaAs and/or an InGaAs graded region to accelerate electrons across the base, and a variety of other modifications may be used to optimize various aspects of performance. However, each of the modifications listed above include a p+ base (layer 3).
During the growth of HBT structures with MBE or OVMPE, the five most likely p-type dopants used are cadmium (Cd), magnesium (Mg), carbon (C), Be and Zn. Be is the preferred choice as the p-type dopant for growing HBTs with MBE, primarily due to its high solubility, good minority carrier lifetime, and low diffusion coefficient under optimized growth conditions. However, Be is not a useful dopant for growing HBT structures with OMVPE because BeO is extremely toxic. For example, the use of diethylberyllium (DEBe) as a p-type dopant for OMVPE GaAs leaves deposits in the reaction cell and exhaust of OMVPE reactors, which must be routinely cleaned. Another reason Be is not useful as a p-type dopant is that the availability of DEBe is currently limited.
The major problem with using Cd as a base dopant is the difficulty of obtaining carrier concentrations above 1.times.10.sup.18 cm.sup.-3. Mg has the advantage of a low diffusion coefficient; however, the use of bismethylcyclopentadienylmagnesium (MCp.sub.2 Mg) as a Mg source has not resulted in abrupt dopant profiles.
The aforementioned problems leave the dopants of Zn and carbon as the primary candidates for OVMPE growth of HBT structures.
To date, the qualification of HBT structures grown with OVMPE has focused primarily on carbon as the base dopant. This is primarily due to the lower diffusion coefficient of carbon compared to Zn and the commonly held belief that this lower diffusion coefficient will yield improved reliability. However, the demonstrated reliability of HBT structures grown with OVMPE using carbon as the base dopant has been significantly worse than those grown with MBE using Be as the base dopant. Thus, the qualification of HBT structures grown with OMVPE using carbon as the base dopant has not been as successful as that grown with MBE using Be as the base dopant.
Furthermore, the use of carbon doping in OMVPE HBT structures causes additional reliability problems and reduced performance (e.g., reduced minority carrier lifetime and accelerated current gain degradation). Research has been performed showing that the lifetime, carrier concentration, and mobility of GaAs HBT structures grown with OVMPE using carbon as an acceptor dopant degrade with post growth annealing. These effects may be due to one or a combination of hydrogen passivation, dopant precipitation, and/or lattice relaxation. Although excessive doping concentrations with carbon does not result in significantly enhanced diffusion, it may result in precipitation. These observations are consistent with the inferior reliability in carbon doped OMVPE HBT structures compared to that observed with Be doped MBE HBT structures.